The basic concept underlying PAL and NTSC format quadrature-modulated color television systems is that the same frequency spectrum is shared by chroma and luminance information (FIG. 1).
For the sake of simplicity, the following presentation is limited to the NTSC color signal format, since it is well understood in the art that the principles hereof apply with like force and result in systems following the PAL signal format.
A typical NTSC encoder is represented by the FIG. 2 block diagram. The FIG. 2 block diagram shows a conventional encoder 10, typically found in association with a color television camera at the studio, or elsewhere. Red (R), Green (G) (sometimes also used directly for the luminance channel) and Blue (B) scanned picture components enter the encoder 10 on R, G and B lines from the camera (not shown). These three components pass through a luminance (Y) matrix 12, an I color component matrix 14, and a Q color component matrix 16. The Y matrix 12 puts out the luminance component Y.
The I color component is passed through a low pass filter 18 having a 6 db cutoff at 1.3 MHz; and the Q color component is passed through a low pass filter 20 having a 6 db cutoff at 0.6 MHz. The filters 18, 20 thereby have the effect of band limiting the I and Q color components. These components enter a quadrature modulator 22 wherein they double sideband amplitude modulate in phase quadrature a color subcarrier (e.g. 3.579545 MHz in NTSC format) which is itself suppressed in level in the modulation process. The subcarrier frequency is selected in such a way as to result in a 180.degree. phase shift from scanning line to adjacent scanning line, and from frame to frame, within the color television picture signal. A burst flag is also added at the quadrature modulator 22.
The luminance signal from the matrix 12 is combined with the quadrature modulated color signal from the modulator 22 in an adder circuit 24 in which synchronization signals such as horizontal and vertical sync, blanking, pedestal, etc. are also added. The resultant composite color signal is low pass filtered in a filter 26 having a 6 db rolloff at 4.2 MHz in order to meet the NTSC signal format standard.
A detailed examination of the frequency spectrum of an NTSC encoded color picture signal in the vicinity of the subcarrier shows the well-known "interleaving" principle (FIG. 3). Spectral rays of a typical television scene are grouped around multiples of Fh (where Fh=horizontal scanning frequency) for the luminance information, while chroma components are grouped around ##EQU1## (where n is an integral number). This grouping is a particularly accurate representation of the spectral appearance of vertical components of the picture, and enables separation of chroma and luminance at the receiver to be accomplished by use of a comb filter with a fair degree of discrimination between these two components.
Thus, FIG. 3 illustrates the conventionally encountered spectral overlap between the luminance and the chroma components located in the spectrum of the composite picture signal put out by the FIG. 2 encoder 10 at the vicinity of the color subcarrier. Substantial spectral overlap is apparent in the FIG. 3 graph and results from imperfect interleaving of the luminance and chrominance spectral components at the vicinity of the subcarrier frequency.
Unfortunately, the frequency interleaving is perfect only in the case of horizontal transitions in the picture. Diagonal and vertical transitions in the picture image manifest an undesirable overlap of chroma and luminance spectra as illustrated in FIG. 3; and, the separation of the luminance and chroma components in the receiver becomes difficult and in some cases, impossible at such transitions.
As a result of imperfect separation of luminance and chroma components at the encoding process, certain luminance components will be misunderstood by the receiver's decoder and decoded as color. This mistake results in the well known "cross-color" pattern which is typically perceived as a moving rainbow accompanying diagonal luminance transitions, or color activity associated with luminance details.
Certain chroma components will also be misunderstood by the decoder and decoded as luminance. One or two lines of dots at the 3.58 MH.sub.z color subcarrier frequency will be perceptibly present in the luminance path for horizontal chroma transitions with comb filter decoders. A decoder using a 3.58 MH.sub.z trap in the luminance path exhibits either a poor frequency response, or a vertical dot pattern with vertical chroma transitions, or both.
Complex decoding techniques using adaptive methods (that is, methods which change the structure of a chroma/luminance separator as a function of the image content) reduce the visual impact of these errors, but circuit implementations of these techniques are costly, and there are always picture conditions where practical limits are reached.
While comb filter structures are known and widely utilized in the prior art, a hitherto unsolved need has remained for practical applications of comb filter structures in the encoding process of conventional quadrature modulated color television systems, such as the NTSC system, and in the decoding process in order to prevent unwanted cross-color and cross-luminance artifacts in the resultant picture display.